This invention relates to a process for producing a 3-layer co-extruded biaxially-oriented polypropylene (hereinafter as BOPP) synthetic paper and transparent film for in-mold label. More particularly, the invention relates to a process for producing a 3-layer biaxially-oriented PP synthetic paper by means of 3-layer co-extruded wherein three different PP resin compositions are separately extruded by one primary and two secondary extruders first, and then are co-flowed by a same T-die to form a 3-layer coating sheet. Thus, the 3-layer structure of said 3-layer co-extruded coating sheet can be made into resin layer/foamed intermediate or resin layer/resin layer, and then through cooling, biaxial orientation, corona treatment and winding to form a 3-layer co-extruded synthetic paper of haze paper sheet layer/formed intermediate layer/adhesive layer, gloss paper sheet layer/foamed intermediate layer/adhesive layer, or a 3-layer co-extruded synthetic transparent film of resin layer/resin layer/adhesive layer.
Typically, the label for use in the container of edible(such as beverage, pickle), pharmacopoeia(such as vitamin, antiseptic), hygienic product(shampoo, body lotion, washing liquid) is the type of back coating label(a product obtained by laminating with releasing paper back coated with adhesive), first it is through the print process, and then adhere the label on the bottle, jar, cap of the container by labeling machine or hand. However, the materials of the label paper depend on the purpose (for example waterproof for bathroom, refrigerator, sink), the recycle operation (for example the scrap of the label paper left on the waste plastic bottles will interfere the recycle operation), and the quality demand (for example the print of the label paper is not fine enough). Nowadays the paper made of nature pulp is replaced by the plastic materials (such as synthetic paper of PVC, PP, PET, PE, etc). When adhering the label by labeling machine or hand, the manufacturer has to buy the equipment for labeling or spends the cost of human, as well as the cost of material and manufacture of the back coating label is enormous. Additionally, the releasing paper after labeling is a composite material consisted of paper and PE with releasing agent, it is only treated as waste and can not be recycled, and increases the load of environment and social cost.
Though, now a synthetic paper for in-mold label is available and has been provided by Nippon Oushi Yuka Company, Ltd., in order to improve the problem of cost and environment of the labeling equipment and the back coating label. The said in-mold label is first pre-treated with dry hot melt adhesive on the rear surface, and printed, positioned in the mold by means of auto-robot attached to the equipment of bottle making (such as blow molding machine, injection molding machine). Then it is integrated with the injection molding plastic material (such as PP, LPPE, HDPE), and using the pre-treated dry hot melt adhesive adhere to the bottles and jars. The surfaces of the finished bottles, jars are smooth, like printing directly on the curved surfaces of the bottles and jars, and it is difficult to feel the existence of the label.
According to tn the above mentioned synthetic paper for in-mold label by Nippon Oushi Yuka Company, Ltd., the manufacturer only need invest costs in the auto-robot and the mold adaptation, the costs of the equipment and human for labeling can be saved. The cost of the synthetic paper with the pre-treated dry hot melt adhesive is twice as many as the cost of back coating label, but it will be benefitted for the long-term user. However for the bottle manufacturer and edible, pharmacopoeia, hygienic product manufacturer, the marketplace is very keen, the sale cycle of the product is not easy to predict, so the manufacturer does not tend to this investment, it is the major reason why the in-mold label cannot be replaced substantially with the back coating label. Only while the market price of the in-mold label is substantially cheaper than the back coating label, the manufacturers are induced to develop in-mold label.
It is an object of this invention to provide a novel process for producing a 3-layer co-extruded biaxially-oriented polypropylene (BOPP) synthetic paper and transparent film for in-mold label.
In view of the foregoing, this invention is intended to obviate the above-mentioned problems, the company which employs the inventors has been devoted to develop the BOPP synthetic paper, and constantly test and research the paper for in-mold label, it is known if the paper for in-mold label was not required pre-treating with dry hot melt adhesive on the rear surface, and could adhere securely to the material of the bottle or jar (such as PP, LPPE, HDPE), such that the production cost would be reduced and enhance the market competence. Naturally, the manufacturers are induced to invest in the auto-robot and the mold adaptation.
Accordingly, this invention has for its object to provide a synthetic paper and transparent film having 3-layer structure, having haze paper layer/foamed intermediate layer/adhesive layer, or gloss paper layer/foamed intermediate layer/adhesive layer, wherein the material of the adhesive layer such as PP, LPPE, HDPE can adhere integrally to the material of the bottle or jar in the mold of injection molding or blow molding, by means of chemical bond, and the hot melt adhesive is not necessary. Therefore the benefit of the product can be shown as follows:
(1) saving the costs using the releasing paper (the paper treated with PP coating film and releasing agent);
(2) saving the costs of the gluing of the releasing paper; and
(3) saving the costs of the auto-labeling equipment or labeling by hand.
The biaxial-oriented polypropylene (BOPP) synthetic paper according to this invention has 3-layer structure, wherein the paper layer having white haze or transparent gloss paper layer suitable for printing, and has printing effect of aesthetic design; the foamed intermediate layer mainly playing the function for significantly decreasing the specific gravity, and providing whiteness and un-transparent of design; and the adhesive layer is used for adhering integrally to the material of bottle and jar in the mold. The 3-layer structure of the biaxial-orientation for in-mold label according this invention is made by means of three extruders in the manner of 3-layer co-extrusion. According to the experience of the applicant for several years, the extruder device can be designated as follows:
(1) Typical single screw extruder: it has to use high-class filler master batch, and the product speed may be restricted.
(2) Single screw extruder of hopper venting type: it has to use high-class filler master batch, but the product speed can be raised a little
(3) Single screw extruder of cylinder venting type: it has to use high-class filler master batch, and both the product speed and the addition quantity may not be restricted.
(4) Twin screw extruder of cylinder venting type with side feeding hoppers: it can use directly powder filler, and both the product speed and the addition quantity may not be restricted.
Basing on the costs and the stable quality of special machine, the production in this company mainly use the twin screw extruder of cylinder venting type with side feeding hopper, but any process according this invention will not apply additionally the adhesive, and the label can be adhered integrally to the container of the bottle and jar.
The biaxial-oriented polypropylene (BOPP) transparent film according to this invention has 3-layer structure, wherein the main material of the upper and intermediate layers is polypropylene or co-polymer of polypropylene; the lower adhesive layer is made by the same material as the bottle and jar, in order to increase integrally adhesive in the mold. Any type of the above mentioned extruder can be combined to co-extrude the 3-layer co-extrusion transparent film, the production in this company mainly use the typical single screw extruder, but any process according to this invention will not apply additionally the adhesive, and the label can be adhered integrally to the bottle and jar.